Nutritional Energetics of Livestock: Some Present …...2008/11/24 · NUTRITIONAL ENERGETICS OF...

J. T. Reid, Ottilie D. White, R. Anrique and A. Fortin

Knowledge and Future Research NeedsNutritional Energetics of Livestock: Some Present Boundaries of

1980. 51:1393-1415. J Anim Sci

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NUTRITIONAL ENERGETICS OF LIVESTOCK: SOME PRESENT BOUNDARIES OF KNOWLEDGE AND FUTURE

R E S E A R C H N E E D S 1 ,2

J. T. Reid, Ottilie D. White, R. Anrique 3 and A. Fortin 4

Cornell University, Ithaca, N Y 14853

Summary

An overview of the nutritional energetics of animals is presented to indicate with a few examples the present state of knowledge and to identify several problems needing research attention. Animals differ from each other in: (1) gastrointestinal architecture, (2) nature and location of digestive agents and (3) major digestive and absorptive sites. These characteristics determine: (1) the chemical and physical nature of the usual diet ingested by a given animal, (2) the rate and extent of digestion and (3) the chemical nature of the absorbed products of digestion; they also influence the efficiency of energy utilization. Both the digestible energy (DE) and the metabolizable energy (ME) values of the usual diets fed in practice and the net utilization of ME for body functions are highest in pigs and fowl, intermediate in horses and rabbits and lowest in ruminants. As the level of input of forage-concentrate diets or finely ground forage increases, the digestibility of energy by herbivores declines progressively. The rate of digestibility depression is especially pronounced in ruminants ingesting high energy diets; this

1 Paper presented at the symposium on "Energetic Efficiency in Producing Animal Food Products," held at the joint annual meetings of the ASAS and the ADSA, Michigan State Univ., East Lansing, July 11, 1978.

2Some of the research cited in this report was supported by funds from Research Grant No. AM- 02889 of the National Institute of Arthritis and Metabolic Diseases, US Public Health Service; the Competitive Research Grant Program, Cooperative State Research Service, USDA, and the Lilly En- dowment, Indianapolis, IN.

3Present address: Instituto de Production Animal, Universidad Austral, Casilla 567, Valdivia, Chile.

4Present address: Animal Research Institute, Agriculture Canada, Ottawa, Ontario, Canada K1A 0C6.

effect seems attributable chiefly to a reduction in gastrointestinal pH, which results in a reduction in the digestibility of starch and cell wall carbohydrates. Improving the DE and ME values of ruminant diets is a major research challenge. The various body functions which utilize ME by mammals within species are, in order of net efficiency, maintenance > lactation > body gain > reproduction. In fowl, the net efficiency of maintenance is a little higher than that of egg production or body gain. A brief summary is made of the influences of nutritional and other environmental conditions and of animal characteristics on the utilization of ME. Evidence indicates that the relation of glucose to acetic acid utilization more nearly explains differences in the net utilization for body gain of ME supplied by different diets to ruminants than does the ruminal ratio of acetic to propionic acid, per se. Among continuously growing animals, the net efficiency with which ME is utilized is high in those gaining fat at a rapid rate and protein at a low rate, and vice versa. The efficiency of the in vivo net synthesis of protein is lower than that of fat, and lower than extant estimates of the theoretical biochemical efficiency of protein synthesis. Research is needed to find a means to divert energy from the synthesis of fat to the synthesis of protein, and yet to improve the energetic efficiency of animals. Under average animal production conditions in the United States, the estimated efficiency with which protein (grams) is produced per megacalorie of DE ingested is as follows for various food products: fish, 40; eggs, 12.6; milk, 12.4; broiler, 11.9; pork, 6.1, and beef, 2.3. These estimates have taken into account the overhead DE costs, such as those of rearing and maintaining the breeding herd or flock, nonlactation periods and the reproductive and mortality periods and the reproductive production itself. The protein outputs increase

1393 JOURNAL OF ANIMAL SCIENCE, Vol. 51, No. 6, 1980


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1394 REID ET AL.

as the intensity of production (chiefly energy input per unit of time) increases, because the energy cost of maintenance becomes a gradu- ally smaller proport ion of the total DE input. Research is needed to identify management components of product ion systems that have the greatest impact on nutrit ional efficiency of livestock. (Key Words: Nutrit ional Energetics, Energy Utilization, Energetic Efficiency, Body Compo- sition, Net P ro t e in Synthesis, Net Fat Syn- thesis.)

I ntroduction

In this paper, it is our intention to provide an overview establishing the present state of knowledge of certain aspects of the nutrit ional energetics of the whole animal. This coverage will also focus on certain research needs which, if fulfilled, are expected to lead to the improvement of the energetic efficiency of farm animals. In addition, the dietary energy costs of protein production by various animal production enterprises will be examined. This study will demonstrate the overwhelming opportuni ty for improving the nutritional energetic efficiency of ruminants as compared with that of simple-gutted animals.

Conditions Affecting the Digestible Energy (DE) and Metabolizable Energy (ME) Values of Diets

Ranges of DE and ME Concentrations in the Diets of Farm Animals. The significance of DE and ME is that they serve universally as the major rationing units or as the bases for com- puting rationing units, and they represent biological indices of the potential nutritive value of the diet. In the study conducted by Moe et al. (1972), 86% of the variation in net energy for milk production among diets consumed by lactating cows was attr ibutable to the concentration of DE in the diets.

The ranges of DE and ME values summarized in table 1 from many literature sources reflect chiefly animal peculiarities and the influence of the quality of diet sometimes ingested by animals. The interspecific peculiarities of animals are at tr ibutable to: (1) the chemical and physical nature of the usual diet, e.g., the ratio of concentrates to forage and the quality of forage (especially cell wall content and the extent of lignification); (2) the architecture of the gut, e.g., location of the major sites of fermentative digestion relative to the major

TABLE 1. RANGE OF DIGESTIBLE ENERGY (DE), METABOLIZABLE ENERGY (ME) AND ME/DE OF DIETS INCREASED BY VARIOUS

KINDS OF FARM ANIMALS

Animal DE ME ME/DE

(% of gross energy) (X 100)

Pigs and chickens 60to98 56to97(80) a 92to98 Horses and rabbits 30 to 90 18 to 85 (70) a 85 to 94 Ruminants 40 to 90 23 to 82 (60) a 78 to 87

avalues in parentheses represent those found in usual good commercial practice.

sites of absorption, an~t (3) the relative degree of dependency on digestion effected by tissue- elaborated enzymes and microorganismal enzymes, and their locations (Reid, 1974). Within species, the differences in the magnitude of DE and ME values are mainly the result of quality of the diet, especially the proportions of soluble cell contents and cell wall substances, and the degree of lignification of cellulose. Extremes in nutrient balance such as very low concentrations of vitamin A or low or high concentrations of dietary protein also may affect the digestibility or metabolizabili ty of energy.

As shown in table 1, the ME/DE values are highest for pigs, chickens and ruminants fed milk, intermediate for nonruminant herbivores and lowest for ruminants. The difference between 100 and these values represents the magnitude of the energy loss from DE in the urine of simple-gutted animals and in the urine and methane of herbivorous animals. The ME values in parentheses represent those of diets usually fed in good commercial practice in the United States. Thus, the ME values of usual practical diets are highest for simple- gutted animals, intermediate for nonruminant herbivores and lowest for ruminants.

As the magnitude of the inter- and intra- specific ranges in the DE and ME values indicates, potentially the greatest impact to be made on the overall energetic efficiency of animals lies in research that will lead to im- provements in the metabolizabil i ty, especially the digestibility, of dietary energy. The opportunity for improvement is particularly great in ruminants.

Effect of Level of Energy Input on tbe DE Value. As the level of energy input provided by a diet of constant composit ion is increased


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NUTRITIONAL ENERGETICS OF LIVESTOCK 1395

per unit of time (e.g., per day), the digestibility of energy varies in a manner that is associated with the gastrointestinal architecture of animals and, within kind of animal, with the chemical and physical nature of the diet (Reid and Tyrrell, 1964). Figure 1 represents a generalization of the influence of the level of intake on the DE value of certain kinds of diets ingested by various farm animals. It is cautioned that the DE values (figure 1) employed, though typical of those found in practice, are merely illustra- tive and are not constants.

In the simple-gutted animals, the level of input has little effect upon the digestibility of energy. Typical of many studies demonstrating this fact is the experiment conducted by Parker and Clawson (1967), in which lactating sows were fed one of two diets at levels of approximately two, four and six times the maintenance intake. However, increasing intakes of highly refined diets, such as one of whole milk that was fed to nonruminating calves (Blaxter, 1952), are accompanied by slightly increasing rates of digestibility. This response probably reflects a diminishing contribution of endoge- nous products to the total fecal matter as the intake level increases.

As the level of intake of concentrate-forage diets by nonruminant herbivores and ruminants increases, the DE value of the diet progressively declines. The influence of the level of input upon the digestibility of energy provided to ruminants by all-forage diets is associated with the particle size of the forage (Blaxter and Graham, 1956). The digestibility is unaffected or affected only slightly when the forage is

90

8O

~o

,~ 6o Q

N 6O

5O

-PIGS & CHICKENS LACTATING SOWS

(CONCENTRATE DIET)

~ H O R S E S & RABBITS

~ 50% FORAGE)

~ R U M I N A N T S (50% CONC, + 50% FORAGE)

RUMINANTS NG OR CHOPPED FORAGE)

RUMINANTS (FINELY GROUND FORAEE)

I ' ' ' " 3 4 5 6

INTAKE (MULTIPLES OF MN'T,)

Figure 1. Effect of level of energy input on the digestible energy value of the diet.

ingested in long or chopped form (i.e., with a particle size of 1 cm or longer); however, the DE value diminishes with increasing inputs when finely ground forage is consumed by ruminants as pellets, meal or gruel. These conclusions are documented by the literature analyzed by Andersen et al. (1959).

The effect of level of intake on the DE value of the diet is especially marked in the high producing dairy cow because of her requisite high intake of feed. Among cows ingesting seven different forage-concentrate diets at levels up to five times the maintenance intake, the digestibility o f energy was de- pressed by 2.1 to 6.2% (average rate 4%) per increment of intake equivalent to maintenance (Moe et al., 1965; Reid et al., 1966) . The total digestible nutrient (TDN) values of corn meal (77%) and ground oats (69%) determined by difference in mixed diets fed to milking cows at approximately 2.5 times the maintenance level were considerably lower than those (91 and 76%, respectively) recorded in tables of feed composition and presumably determined at about the maintenance level with nonlactating ruminants (Moe et al., 1973). If these differences are attributable to the intake level, the depressions in TDN per multiple of maintenance were 5% for the corn diet and 2.2% for the oat diet.

Reid and Tyrrell (1964)called attention to the observation that the rate of decrease in digestibility that accompanies increasing intakes, though variable, is more marked for high concentrate diets than for diets containing less concentrates. They also suggested that the rate of depression in digestibility generally increases as the ratio of propionic acid to acetic acid produced in the rumen increases. Recently, Tyrrell and Moe (1975) examined the inter- action between the intake level and the proportion of concentrates in the diet and confirmed that the rate of depression in digestibility is greater for high concentrate diets than for diets high in forage. In their generalized relationship, the rate of digestibility depression was .052% for each 1% increase in grain content of the total diet.

Although much more research is needed to determine the fundamenta; causes of the depression in digestibility associated with the intake level and to quantify their effects, present information indicates that digestibility depression is influenced by certain conditions associated with the environment, particularly


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1396 REID ET AL.

the pH, of the gastrointestinal tract. For example, in lactating cows fed mixed diets containing 50% of ground corn or ground barley, the reductions in digestibility (per multiple of maintenance energy intake) of certain dietary fractions were: hemiceUulose, 8.2%; cellulose, 8.1%, and soluble cell contents, 3.2% (Tyrrell and Moe, 1972). In studies of the effect upon the DE in diets of various concentrate-forage ratios fed at various levels of input, Wheeler et al. (1975) observed that the digestibility of starch ranged from 84.7 to 88.1% at intakes of 2.5 to 3.2 times maintenance, but that it was 96.2 to 96.8% at the maintenance intake level. In subsequent studies, Wheeler and Noller (1977) found that the pH of the ingesta from the rumen to the rectum (except in the abomasum) and that of the feces are on the order of 5.6 to 5.8 in cattle fed high energy diets. The correlation coefficients between fecal pH and the amount of fecal starch were - . 8 2 and - . 9 4 for two different diets. The addition of limestone or magnesium limestone to the same diets increased the pH of the intestine by about 1 unit and reduced the amount of starch in the fecal dry matter from about 32 to 9%. These workers suggested that the improved digestibility of starch might have occurred because the increased pH of the small intestine provided a more favorable environment for the action of pancreatic a-amylase, although increased pH may also have improved ruminal digestibility.

Wheeler (1977) reported that the addition of 2.17% of limestone to a complete diet fed to milking cows at 3.5% of live weight improved the digestibility of the following fractions: energy (from 65.7 to 69.0%), starch (from 85.9 to 94.6%), cell wall constituents (from 51.3 to 55.1%) and crude protein (from 65.7 to 67.8%). In generalizing the data from Purdue on the depression in digestibility associated with increased intakes of high energy diets, Noller (1978) indicated that the following proportions of the total reduction in digestibility are attributable to decreases in the digestibilities of these dietary components: starch, 45%; cell wall components, 45%; protein, 8%, and others, 2%. However, the contribution of each to the total depression in digestibility depends on the nature of the diet.

Ruminants performing high demand functions, such as high milk production or rapid body gain, require large energy inputs per unit

of time. Generally, these inputs are achieved through the feeding of high concentrate diets. With increasing inputs of such diets, the digestibility of energy declines progressively. As intake increases, the rate of ingested passage increases. Since fermentative digestion is relatively slow, the extent of cell wall digestion is reduced. High concentrate diets result in the production of large amounts of volatile fatty acids, which may reduce the pH of the rumen and small intestine to within 5.0 to 6.0. (Yet, in ruminants on all-forage diets, the pH may range up to 7.0.) At a pH of less than 6.0, the growth of cellulolytic bacteria is inhibited (Hungate, 1966; Slyter et al., 1970). Hemi- cellulase prepared from rumen liquor of sheep fed a hay diet reached maximum activity 5 hr after feeding; the enzyme complex was most active at pH 7.0 (Morrison, 1976). The pH optima for the following carbohydrases are: pancreatic a-amylase, 6.9; intestinal maltase, 6.8 to 7.0, and oligo-l:6 glucosidase, 6.2 to 6.4 (data from various sources summarized by Arm- strong and Beever, 1969). Thus, it is indicated that the gastrointestinal environment of ruminants ingesting high levels of high concentrate diets would impair the activity of enzymes re- sponsible for the digestion of cellulose, starch and other nutrients. This could account for most of the depression in the digestibility of energy that accompanies increased energy intake.

Other Research Needs Associated with Energy Digestibility and Metabolizability. Methods and criteria of chemical and physical definition are needed to refine the prediction of the DE and ME values of diets at different levels of input. Also, methods of estimating the net nutritive effects of the DE and ME of various diets are needed as rationing aids.

Depending on the nature of the diet and the level of feeding, ruminants lose as com- bustible gases 5 to 12% of the dietary energy ingested (Blaxter and Clapperton, 1965). Al- though methanogenesis is a wasteful process, its inhibition would be beneficial only if other processes such as digestibility of energy, synthesis of microbial protein and accumulation of hydrogen were not inhibited to the same extent (Czerkawski, 1971, 1972, 1974). Many compounds (e.g., chloroform, methylene chloride, carbon tetrachloride, long-chain fatty acids, bromochloromethane, hemiacetyl of chloral hydrate, 2, 2, 2-trichloroacetamide, trichloro-

l adipate and others) are known to suppress lane production (Bauchop, 1967; Clapper-


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ton, 1977). The same compounds invariably influence the fermentation pattern, increasing the proportion of propionic acid and reducing that of acetic acid. Thus, two advantages could result from the suppression of methanogenesis: the reduction in energy loss as methane, and an increase in the efficiency of utilization of ME provided by diets that do not result in sufficient glucose precursors.

To this stage, the energy partition and growth and lactation responses of animals given methane suppressants have not been uniform. In calorimetric experiments with sheep fed 2.2 g/day hemiacetyl of chloral and starch (HCS) in a mixed diet containing 40% hay, CH4 loss was reduced by 4.4% of the gross energy (GEL but H2 production was increased by 1.65% of the GE; thus, as a percentage of the dietary GE, the net loss of gaseous energy was reduced and the ME was increased by 2.75% over that of the control diet (Johnson, 1972). On the other hand, HCS did not affect the utilization of ME for maintenance or body gain. Clapperton and Czerkawski (1972) infused 440 mg of chloroform into the rumens o'f sheep as the animals ate 1,000 g/day of hay in two equal meals. In control periods, only the hay was fed. Chloro- form did not affect the energy loss in the feces or urine, but it reduced the CH4 production from 23.8 to .9 liters/day and increased the production of H2 from 0 to 17.7 liters/day. The net intake of ME by the sheep receiving the chloroform was 163 kcal/day greater than that of the sheep not given chloroform. Since heat production was 109 kcal/day greater for the chloroform-treated sheep, indicating a Kf value of .33, it is unlikely that suppression of methane production improved the efficiency of utilization of the ME.

In feeding trials, methane depressants have resulted in both negative and positive responses. Sheep given 2, 2, 2-trichloroacetamide (Trei et al., 1971) or HCS (Trei et al., 1972) grew more rapidly and converted feed 3 to 7% more efficiently than did the controls. Opposite effects were observed in sheep given bromochloromethane (Sawyer et al., 1974) or trichloroethyl adipate (Clapperton, 1977). Also, no improvement in rate of body weight gain or feed efficiency of cattle was effected by bromochloromethane (Johnson et al., 1972). Chloroform administered by rumen cannula to lactating goats did not affect the milk yield; however, it reduced the fat concentration and increased the protein content of the milk (Clap-

perton and Basmaeil, 1976). Although the suppession of methane pro-

duction and the conversion of hydrogen to energy-conserving substances would represent a substantial gain in ME, little gain of practical value has been effected to date. In some studies, methane suppressants have reduced feed intake. Animals adapt to some antimethano- genic agents by returning to normal methane outputs a few weeks after the initial suppression (Clapperton, 1977). The benefits of methane suppressants for growing-fattening animals may be different from those for lactating animals.

Dry matter digestibility by ruminants is somewhat lower under cold ambient temperatures than it is undet; higher temperatures. In a summary of 11 experiments carried out in cold regions of Western Canada, Young (1976) reported an average decline in digestibility of .18 percentage units per degree decrease in temperature below 20 C at the same level of feed input. In some experiments, there was little or no effect. The reduction in digestibility was attributed to an increased rate of ingestal prssage through the gastrointestinal tract, and Kennedy et al. (1976) observed that digestibility of dry matter in the intestines accounted for more of the apparent digestion in the whole gastrointestinal tract of sheep kept in the cold than it did in sheep kept at higher temperatures. Also, the reduced time of retention of ingesta in the rumen and the decreased digestibility of energy observed at low temperatures are associated with increases in the concentration of thyroid hor- mone in the blood plasma (Kennedy et al., 1977). W/Shlbier and Schneider (1965) observed no effect of temperature on the digestibility of energy by steers over a range of 10 to 30 C.

Efficiency of Utilization of ME by Farm Animals

General M o d e l o f E x a m i n a t i o n . The model by which certain kinds of influences on the utilization of ME will be examined is represented in figure 2. This examination of animal energetics employs the well-established linear- ity of the relationship between the ME intake (X) above maintenance and the resulting energy balance (Y), i.e., energy gained by the body or put into other products. Both of these variables will be expressed in kilocalories per kilogram of body weight raised to the power .73 or .75, and will sometimes be referred to as


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1398 REID ET AL.

~ O N (Z))

I r

MAINTENANCE METABOLIZABLE ENERGY INTAKE

Figure 2. General relationship of metabolizable energy intake to energy balance.

metabolic body size (MBS). The relationship between these two variables provides: (1) the slope, which is the index of the efficiency with which ME is utilized to support a given function such as body gain, lactation, egg production or reproduction, and (2) the ME-intake intercept at zero energy storage; this value is the ME requirement of maintenance. In using this scheme to partition the net efficiency of one body function when several body functions are being performed simultaneously, it is necessary to deduct from the total energy balance the energy gain represented by the functions not being examined and to subtract the ME input associated with the functions not being investigated from the total ME input. However, much further research is needed to refine requisite coefficients for partitioning partial efficiencies in animals performing two or more functions.

Require~nent o f ME for Maintenance and the Net Utilization o f ME for Certain Production Functions. The efficiency with which ME is utilized is influenced by, or associated with: (1) environmental conditions, especially temperature, humidity and wind movement; (2) extent of physical activity; (3) sex; (4) gastrointestinal architecture and major digestive agents; (5) the nature of the absorbed products of digestion; (6) the body functions being performed, and (7) a variety of dietary characteristics. For example, insufficient amounts of protein and of certain vitamins and mineral elements in the diet, along with an imbalanced assortment of absorbed amino acids, reduce the efficiency with which ME is utilized by animals for maintenance and production.

Since a detailed account of some of these influences and associations is given elsewhere (Reid, 1974; Reid and White, 1978a), only a few examples will be cited here to indicate the present boundaries of progress.

The partial efficiencles with which ME is utilized to support the individual body functions, in combination with the ME value of the diet, provide the means to derive by the factorial method feeding guides that might be applied under different pi~ctical conditions. In table 2, a summary is given of the maintenance requirements and the efficiency with which ME is utilized to support certain production functions of several animals fed diets commonly used in practice. The summary includes values rounded from those reported by Reid (1974) and Reid and White (1978a), but also contains data for immature mink (Harper et al., 1978), 10- to 25-day-old chicks (Thomas, 1966), 4- to 10-kg pigs (Close et al., 1979) and laying hens (Waring and Brown, 1967). These data reflect a higher maintenance requirement of very young animals than of older animals (e.g., among mink, chickens, pigs and calves), of milking cows than of nonlactating cows and of laying hens than of mature, fattening cockerels. Differences attributable to degree of maturity also exist in other species.

The maintenance requirement of ME increases with: (1) increasing physical activity, e.g., grazing animals require 10 to 90% more ME than do ruminants confined to stalls (Reid et al., 1958; Graham, 1964a,b; Young and Corbett, 1972), and increasing gradients increase the energy cost of locomotion (Clap- perton, 1964; Ribeiro et al., 1977);(2) ambient temperature below or above the zone of thermal comfort (Joyce and Blaxter, 1964; Shannon and Brown, 1969; Nehring, 1958); (3) increasing amounts of dietary protein above the requirement (Garrett, 1970; Tyrrell et al., 1970), e.g., in dairy cows, ME expense of metabolizing amino acids ingested in excess of the protein requirement is 7.2 kcal/g of digestible nitrogen (Tyrrell et al., 1970); and (4) decreasing metabolizability of dietary energy (van Es and Nijkamp, 1967a,b; Moe et al., 1970). The range in ME requirements for each kind of animal listed in table 2 reflects both the quality of the diet ingested and the productive function being performed.

The data in table 2 also indicate that the net efficiency with which ME is utilized for various


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N U T R I T I O N A L E N E R G E T I C S O F L I V E S T O C K 1399

T A B L E 2. R E Q U I R E M E N T O F M E T A B O L I Z A B L E E N E R G Y (ME) F O R M A I N T E N A N C E A N D T H E N E T U T I L I Z A T I O N O F ME F O R C E R T A I N P R O D U C T I O N

F U N C T I O N S BY V A R I O U S A N I M A L S

Animal

Maintenance requirement of ME, Body

kcal/MBSa/day gain, %

Net u t i l i z a t i on o f ME a b o v e m a i n t e n a n c e fo r :

Lactation or egg production, %

Mink , 20% m a t u r e 1 4 8 71 . . . Pigs, 4 t o 10 kg 1 4 8 8 3 Pigs, 1 0 0 to 1 8 0 kg 85 t o 1 2 0 75 to 85 7 5 " t o 85 d C h i c k e n s , 10 t o 25 d a y s o ld 1 7 0 8 0 . . . Chickens, older 85 b to 120 c 75 to 85 80 to 85 Calves, 16 to 50 days old, milk-fed 145 85 . . .

Guinea pigs 85 to 90 60 to 70 70 to 75 d Rabbits 85 to 110 60 to 70 70 to 75 d Horses 85 to 110 60 to 70 70 to 75 d

Sheep, growing or milking 75 to 100 30 to 65 e 30 to 70 Cattle, growing 100 to 125 30 to 65 e Cows, milking 125 30 to 75 30"to 70 Cows, nonlactating 100 30 to 65 . . .

aMBS = metabolic body size; body weight in kilograms raised to .73 or .75 power.

bMature (3.5 to 4.8 kg) fattening cockerals.

CLaying hens.

dEstimated values.

eHigher values were obtained for females; lower values were found for males fed the same diets.

production functions when usual diets are ingested is highest in simple-gutted animals such as pigs, chickens and milk-fed calves; intermediate in nonruminant herbivores such as guinea pigs, rabbits and horses, and lowest in ruminants. (Although not shown in table 2, the efficiency of ME utilization for maintenance ranks in the same order for various animals.) For each kind of animal, the range of net efficiency with which ME is utilized reflects the quality, especially the composition and metabolizability, of diets sometimes ingested in practice. For example, for ruminants, the highest values represent high concentrate diets and the lowest values represent all-forage diets of medium quality. The net utilization of the ME of some forages is even lower; e.g., only 8 to 17% of the ME of wheat straw ingested above maintenance is utilized by cattle for body gain (Kellner, 1900). In one experiment (Blaxter and Wainman, 1961) in which the same diet was fed to mature wethers and steers, the net efficiency with which ME was utilized for body gain was the same for the two species (53.6 and 54.6%, respectively); however, the ME requirement for maintenance

of the wethers (74.4 kcal/MBS/day) was considerably lower than that of the steers (106.4 kcal/MBS/day).

Another example of the effect of dietary composition involves the concentration of fat. Increasing proportions of dietary fat at the expense of carbohydrates have resulted in increased rates of retention of ME in the bodies of rats, chickens and sheep (Reid and White, 1978a). Undoubtedly, the higher energetic efficiency of high fat diets is the result of direct incorporation of dietary fatty acids into triglycerides. This direct incorporation circumvents the energy cost of de novo synthesis of fatty acids from other dietary sources, a process requiring reduced nicotinamide adenine dinucleotide phosphate.

The partial efficiency of ME utilization is associated with the body function being performed; that of lactation is greater than that of body gain except when fattening accompanies lactation (table 2). Although not shown in table 2, the apparent partial efficiency of maintenance is greater than that of production functions. The net efficiency of utilization of the available ME for repro-


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1400 REID ET AL.

duction (i.e., energy retention in the conceptus) in the cow and ewe ranges from 10 to 25% (van Es, 1961; Brockway et al., 1963;Graham, 1964c; Hashizume et al., 1964; Moe et al., 1970; Moe and Tyrrell, 1972; Garrett et al., 1976).

Although the ambient temperature and the extent of physical exercise have pronounced effects on the amount of ME required for maintenance, they do not influence the degree to which ME ingested above maintenance is used for production functions.

As the data in tables 1 and 2 show, both the ME values of the usual diets and the efficiency with which ME is utilized for productive functions are lowest in ruminants. Accordingly, there is much greater opportunity to improve the energetic efficiency of ruminants than of other animals, particularly the simple-gutted ones. Therefore, the ensuing paragraphs of this review will concern subjects that are significant especially to the energetics of ruminants.

Relation o f Metabolizability o f Energy to tbe Net Utilization o f ME. The proportion of the dietary GE that is ME (i.e., ME/GE, sometimes denoted as "q," the quality factor) influences the net efficiency with which ME is utilized for various body functions. Figure 3 depicts this effect. Although the effect of the metabolizability of energy on net utilization is probably different for the individual body functions (Moe and Tyrrell, 1975), van Es (1976) proposed as a compromise that, as the ratio of ME/GE increases by 1 percentage unit, the requirement of ME for maintenance, milk production and body gai n in cattle decreases by about .4%.

End Products o f Digestion. For 80 years preceding 1957, the cause of the lower energetic efficiency of ruminants than of simple- gutted animals remained a mystery, and it is not wholly settled even yet. yon Mering and Zuntz (1877) and others attributed the ruminant's high heat increment to the "work of digestion." Since the early 1900's, it has been recognized that the productive value of the ME provided by starch is considerably higher in the simple-gutted animals than in the ruminants. Data illustrating this difference are summarized in table 3.

Eventually, the viewpoint developed that the high heat increment of ruminants is associated with the volatile fatty acids (VFA) produced in the rumen. In experiments dealing with lipogenesis, Armstrong and Blaxter (1957,

~ 0 I I I I I f I i

M M M M M M M M

RELATIVE ME INTAKES PROVIDED BY DIETS

WHICH ARE:

HIGHLY . POORLY METABOLIZABLE ( ~METABOLIZABLE

Figure 3. General relationship of metabolizability of dietary energy to the metabolizable energy requirement for maintenance and the retention of energy.

1961) and Armstrong et al. (1958) infused acetic acid, propionic acid and butyric acid, singly or in mixtures, into the rumen of sheep fed a maintenance diet of hay. The net efficiencies with which the ME of the VFA infused singly was utilized for body energy gain were: acetic acid, 32.9%; propionic acid, 56.3%, and n-butyric acid, 61.9%. The ME of mixtures of the VFA containing molar proportions of 75 and 25% acetic acid (and of 15 and 45% propionic acid, respectively) was utilized with a net efficiency of 31.8 and 58.1%, respectively. When glucose was infused into the rumen, abomasum or a jugular vein, Armstrong and Blaxter (1961) found the net efficiencies to be 54.5, 71.5 and 72.8%, respectively. Thus, fermentation in the rumen reduced the net efficiency of glucose utilization. When its fermentation was circumvented by adminis- tration into the abomasum or jugular vein, the net utilization for energy gain in sheep was similar to that (73.7%) observed by Nehring (1961) in rats fed glucose. As a conse- quence of such observations, the viewpoint developed that increased proportions of propionic acid in relation to those of acetic acid in the ruminal mixture of VFA result in increased efficiency of ME utilization for body gain. Also, it seemed that the difference in re- sultant VFA mixtures might explain the greater net efficiency of ME provided by concentrates than of ME supplied by forages.

However, many experiments conducted since 1965 have failed to confirm that the ruminal ratio of acetic to propionic acid per se influences the net utilization of ME for body energy gain. The results of one such experiment


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TABLE 3. UTILIZATION OF METABOLIZABLE ENERGY OF STARCH FOR BODY GAIN

BY VARIOUS ANIMALS a

Species

Utilization of ME for

body gain, %

Rat 75.5 -+ /.2 Pig 75.5 -+ 1.6 Rabbit 67.0 -+ 1.2 Sheep 64.1 -+ 1.6 Cattle 64.1 -+ 1.7

aData of Nehring et al. (1961) and Jentsch (1967).

(Bull e t al., 1970) are summarized in table 4. It should be noted that neither the net utilization of ME for body gain nor the maintenance requirement was different at two different acetic to propionic acid ratios with each of two basal diets ingested by sheep. Certain characteristics of that experiment (Bull e t al., 1970) were different from those of some of the other studies: (1) the hay was of very high quality (74% DE); (2) both diets were finely ground and pelleted, and (3) acetic acid was administered as triacetin, thus providing glycerol, and glucose precursor. In a recent study, Orskov e t al. (1978) infused two levels of each of six mixtures of VFA into the rumen and administered protein into the abomasum of sheep receiving no basal feed. On a molar basis, each VFA mixture contained 10% butyric acid. The molar ratios of acetic to propionic acid ranged from .64:1 to 17:1. The net utilization of ME for body gain ranged from 60 to 65%

for ratios ranging from 1:1 to 17:1. At the lowest acetic acid concentration, the net efficiency was 75%. The maintenance requirement of ME did not differ between treatments, and the mean value for all treatments was 103 kcal/kg'75/day. It was concluded (Orskov e t

al., 1978) that since the VFA proportions resulting from practical diets are not outside the range of 1:1 to 17:1 ratios of acetic to propionic acid, differences in the utilization of ME cannot be explained by differences in the efficiency with which VFA are utilized.

It is well established that the net utilization for body gain of ME from pelleted, finely ground forage is higher than that of ME provided by forage of the same source fed in chopped or long form. Also, the ME requirement for maintenance is somewhat lower for animals ingesting forage of small particle size than it is for those fed chopped or long forage. Some typical results demonstrating these effects, as reported by Paladines e t aL (1964), are summarized in table 5. Since it is well established that the feeding of forages of small particle size results in a ruminal mixture that contains a higher proportion of propionic acid and a lower proportion of acetic acid than do mixtures resulting from the feeding of forages in chopped or long form, it had seemed that the mixture of VFA produced might be accountable. However, Thomson e t al. (1972) and Beever e t al. (1972) observed that 23 to 51% more of the energy of finely ground forage than of chopped forage of three sources escaped from the rumen and was digested in the small intestine. These observations imply that more carbohydrate might be absorbed as

TABLE 4. UTILIZATION OF ACETIC ACID FOR BODY GAIN BY SHEEP a

Basal diet Additive

Ruminal Maintenance Net C a :C 3 requirement, utilization ratio b kcal/MBS/day of ME, %

Hay, Triacetin d 5.4:1 88 60 pelleted c Glycerol e 3.2 : 1 86 60

Hay and corn, Triacetin d 5.2 : 1 88 64 pelleted Glycerol e 2.9 : 1 85 62

aData of Bull et al. (1970).

bc 2 = acetic acid and C 3 --- propionic acid.

CEarly-cut alfalfa; DE value was 74% at the maintenance level of intake. d . Tncacetm provided 10.2% of dietary GE.

eGlycerol provided 1.8% of dietary GE.


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1402 REID ET AL.

TABLE 5. EFFECT OF PHYSICAL FORM OF FORAGE AND DIET COMPOSITION ON THE ENERGETIC EFFICIENCY OF GROWING SHEEP a

Diet

Maintenance DE at requirement, maintenance, % kcal/MBS/day

Net utilization of ME, %

Hay b, chopped 60 92 31 Hay b, ground and pelleted 58 87 43

50% hay b +50% corn, 73 82 57 ground and pelleted

aData of Paladines et al. (1964).

bltay of same source in all diets.

simple sugars rather than as VFA from ground forage than from chopped or long form.

It is difficult to assess the proport ion of the heat increment that is ascribable to fibrousness or physical form of the diet. However, Osuji et al. (1975) observed that the energy costs of eating (kilocalories/kilogram of dry matter ingested) were, 57 for chopped, dried grass and 5.7 for pelleted, dried grass. Some of the same workers (Webster et al., 1976) showed that the total costs of eating, comminuting, fermenting and processing the ingesta in the gut constitute 25 to 30% of the total heat increment, but that the energy costs of work by gut tissue are not different for the two physical forms. Earlier, Webster et al. (1974) had found that the "work of digestion" is high for long or chopped forages.

Tyrrell et al. (1976) conducted a study to determine the effect of the nature of the basal diet on the utilization for lipogenesis of acetic acid infused intraruminally into mature, nonlactating cows. Two basal diets, consisting of hay alone and 30% hay with 70% concentrates, were used. In one experiment, both basal diets were fed "~ in ground, pelleted form, and in another, the hay was fed in long form and the concentrates in meal form. The efficiencies with which the ME provided as acetic acid was utilized for body gain are summarized in table 6. These data show a markedly higher utilization of the ME of the acetic acid increment for the high concentrate basal diet than for the medium quality hay basal diet. Qualitatively similar observations had been made previously in a study of growing lambs fed high and low concentrate diets (Poole and Mien, 1970).

In another experiment with growing heifers

and nonlactating cows fed high forage diets (Tyrrell et al., 1977a), acetic acid was infused into the rumen or glucose was infused into the abomasum, or both were infused simultaneously. The simultaneous addition of glucose into the abomasum effected a marked improvement in the utilization of the ME provided by the acetic acid. When no glucose was administered, the utilization of acetic acid infused intraruminally was essentially nil. Stokes and Thomas (1977) also indicated that the difference in the net utilization of ME for fattening is linked with a greater uptake of glucose from the small intestine of sheep fed high concentrate diets than from that of sheep fed a high forage diet.

The results of these studies are consistent with the hypothesis that glucose is required for the efficient utilization of acetate for lipogenesis as the result of glucose oxidation via the pentose phosphate cycle to yield reduced nicotinamide adenine dinucleotide, which is required to lengthen fatty acid chains. Al- though the observations of Tyrrell et al. (1976, 1977a) and Stokes and Thomas (1977) clarify the disparate results of many of the experiments conducted since 1957, those of Orskov et al. (1978) seem enigmatic unless sufficient glucose was produced from certain amino acids, butyrate and propionate to maintain a high level of acetate utilization.

Assoc ia t ion o f S e x o f R u m i n a n t s w i t h Ener- get ic E f f i c i ency . In the study by Bull et al. (1970), in which rams and ewes were fed each of two diets, the maintenance requirement of ME did not differ between sexes, but the net efficiency with which ME was utilized for body energy gain was decidedly higher for ewes (67%) than for rams (57%, table 7). From 20.


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NUTRITIONAL ENERGETICS OF LIVESTOCK

TABLE 6. EFFECT OF BASAL DIET UPON THE UTILIZATION OF ACETIC ACID FOR BODY GAIN BY NONLACTATING COWS a

1403

Exp. no. Hay

Basal diet of:

30% hay + 70% concentrates

1 b 2 c Mean, weighted

Utilization (%) of ME from acetic acid 24.2 81.8 28.1 56.6 26.6 68.7

aData of Tyrrell et al. (1976).

bBoth basal diets fed in pelleted, ground form.

CHay fed in long form and concentrates in meal form.

to 50 kg, ewes contained 30 to 35% more fat and 3 to 17% less protein than did ' rams. During the 175-day feeding period, ewes gained 30% more fat and 31% less protein than did rams. Similar results were obtained in studies with heifers, bulls and steers of two breeds of cattle (R. Anrique, H. Ayala, S. Simpfendorfer, A. Fortin, A. F. Kertz, J. T. Reid and G. H. Wellington, unpublisbed data). The greater leanness of bulls than steers was associated with a significant increase in daily loss of ME as heat, even though the bulls and steers were pair-fed (Webster, 1977).

The energetic efficiency and carcass composition of steers pair-fed a basal diet or a diet containing diethylstilbestrol (DES) were examined by Rumsey et al. (1977) and TyrreU et al. (1977b). At the end of the 27-week feeding period, they observed that the gain in carcass of the DES-fed steers consisted of 33% more protein, 76% more ash, 4% more energy, but 1% less fat than did that of t h e

TABLE 7. RELATIONSHIP OF SEX TO ENERGETIC EFFICIENCY OF

SHEEP a

Maintenance Sex requirement of ME, Net utilization Sheep kcal/MBS/day of ME for gain b

Rams 88 57 Ewes c 87 65

aData of Bull et al. (1970).

bDuring 175-day feeding period of a slaughter experiment.

CEwes gained 30% more fat, but 31% less protein than rams.

controls. As shown in table 8, despite the larger carcasses and protein gain of the DES-fed steers, the util ization of ME for body energy storage by the DES-fed steers (40.8%) was lower than that by the controls (50.6%).

Some Research Needs Concerning Whole- body Energetics o f Animals. Continued examination is needed to determine the exponents of body weight that are most effective to express energetic data for various ages and kinds of animals and conditions. The literature cites exponents ranging from .5 to 1.0. In 1964, the Energy Metabolism Symposium (Kleiber, 1964) adopted body weight .75 as the reference base for comparing the metabolism of various animals. This practical compromise was based on the proposals of Kleiber (1932) and Brody and Procter (1932), who computed the exponents of best fi t between the interspecific basal metabolic rate of mature animals and their body weights to be .75 and .734, respectively. How- ever, it is recognized that a value approaching .5 may be more precise for growing animals, group-confined animals and animals with little thermal insulation. Also, the body weight value employed and the maintenance of conditions under which basal metabolic rate may be determined are fraught with many problems relative to interspecific uniformity. Ideally, although seldom practical, the power function would need to be determined for each experi- mental situation, especially if the size of the subjects were appreciably different.

Thonney et al. (1976) showed that weight exponents are not equal for species-sex-source, sex-source within species or species-source within subclasses. Thus, a universal equation relating basal heat product ion to body weight cannot be considered the best fit for species-


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1404 REID ET AL.

TABLE 8. EFFECT OF DIETHYLSTILBESTROL (DES) ON THE CARCASS COMPOSITION AND

ENERGETIC EFFICIENCY OF STEERS a

Utilization CarcassC Treat - of ME ment b for gain, % Weight Protein Fat

(g) Control 50.6 240 37.6 73.8 DES 40.8 258 42.2 73.4

aData of Rumsey e t al. (1977) and Tyrrell e t al. (1977b).

bDiet contained 70% concentrates and 30% forage; DES-treated group was fed 20 mg of DES per day; animals in the two groups were pair-fed to gain about 1 kg/day for 27 weeks.

Ccarcass characteristics at end of feeding period; the gain in the carcass of the DES steers consisted of 33% more protein, 76% more ash, 4% more energy, but 1% less fat than in that of the controls.

sex subclasses. The authors also felt that within species, there is no justification for use of the allomed'ic equation (i.e., u = aXb). In fact, a simple linear equation (i.e., Y = a + bX) often fits the data more closely because it allows for an intercept, a necessary attribute for the examination of data that do not approach the ordinate (0, 0) along the abscissa (X).

Additional experimentation of a quantita- tive nature is needed to refine the energy maintenance cost and the utilization of ME for production functions within species as influenced by breed, sex, age, climate, physical activity, hormonal status, thermal insulation, body composition, nutrient concentrations, absorbed products (rather than disappeared substances) and stage of growth and lactation. It is proposed that such experiments be conducted at two or more levels of energy input above maintenance. In experiments in which the estimation of the maintenance cost is the objective, it is suggested that heat production be determined by respiration calorimetry so that an almost instantaneous measurement can be obtained. Because of the relatively long feeding period required, the comparative slaughter method is subject to varying climatic conditions and changes in animal maturity and physiological state.

From the standpoint of application, model- ing and the "systems" approach need to be applied to animal energetics in order to inte-

grate the great array of input components, some of which are changing at different rates and in various directions. Such an approach applied to current knowledge might well identify: (1) lack of present information, (2) aspects of animal energetics that have the greatest impact, (3) those aspects that need further quantification and (4) means of generalizing and simplifying to facilitate practical feeding. This approach might help to direct animal energetics into the "real world," which oftentimes must operate outside the range of thermal neutrality or other confined conditions.

Relation of Body Composition to Energetic Efficiency

Among animals undergoing sustained, unin- terrupted growth, those that are depositing fat at the most rapid rate have the highest efficiency of body energy storage and the lowest efficiency of weight gain per unit of ME input. Within species, the amounts of body fat and protein at a given body weight vary among breeds depending on their maturation rates. Early-maturing breeds of ruminants have more fat and less protein than do late-maturing breeds of the same body weight. Within breeds, the propensity to fatten is associated with sex (table 7; figure 4), which is probably a reflec- tion of the kinds of hormones and the syn- chrony of hormonal action that influence the maturing rate. Among ruminants, the female matures earlier and contains more fat at the same body weight than does the male with intact gonads. The male castrate is intermediate in these characteristics. Other animals, such as fowl and pigs, are different from ruminants. For example, body compositions of young male and female pigs (.9 to 30 kg of body weight) with intact gonads were not different at the same weights (Wood and Groves, 1964). Although the differences are not large beyond 30 kg, male castrate pigs have a little less fat at body weights below 70 kg and a little more fat above 70 kg than do females of the same body weights (Reid e t al., 1968). At 100 kg, barrows contain about 2.2 kg more fat than do gilts.

Many reports have been published claiming that body composition has been influenced by certain nutritional treatments. In some in- stances, the conclusions have been correct; however, in others, it is clear that the effects of


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i00

90

80

70

~- 6O

5O

4O

3O

20

180

160

--- 140 v ---120

"I00 >-

80

60

40

20

B = BULL f B ~ B S = STEER ~ S ~ H = HEIFER jP j ~ j S

, H

f AD LEIVBEL OF INP f j UT

70% AD LIB . . . .

H

S~S" .

I I I I J 150 250 350 450 550

EMPTY BODY WEIGHT (KG) Figure 4. Relationship of sex and level of energy input to the body composition of Holstein cattle.


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1406 REID ET AL.

the dietary treatments on body composition have not been independent of the effects of the treatments on total body weight. In some experiments, animals were fatter, and thus contained different quantities of other chemical components, chiefly because they were larger than the animals with which they were compared.

In the following paragraphs, evidence will be presented that indicates that rapid fattening and low protein storage are associated with a high energetic efficiency in animals growing uninterruptedly. Also, an anomaly to this generalization that occurs in compensatory growth will be considered.

Rate of Fattening and Protein Deposition, and Energetic Efficiency. Data in table 7 show that the energetic efficiency and rate of fat gain is lower in ewes than in intact rams. In Holstein cattle fed the same diet ad libiturn or at about 65% ofad libitum, the order by sex of the amounts of body fat at the same body weights and feeding levels was heifers > steers > bulls, as shown in figure 4 (R. Anrique, H. Ayala, S. Simpfendorfer, A. Fortin, A. F. Kertz, J . T . Reid and G . H . Wellington, unpublisbed data). The order of the efficiency of body energy gain was the same, but the order of the amounts of body protein was opposite to that of body fat. In that study, the disconcerting observation was made that the dietary energy input influenced the body composition at the same body weights; i.e., more fat and less protein were stored in animals ingesting the higher level of energy than in those receiving the lower level. This observation is different from those made by Guenther et al. (1965), Stuedemann et al. (1968), Murray et al. (1975), Swan and Lamming (1975) and Jesse et al. (1976). Those authors concluded that the level of energy intake above maintenance has little effect on body composition of cattle. However, Andersen (1975) reported that cattle of the same body size fed ad libitum contained progressively more separable fat and less muscle than did their mates fed at 85, 70 or 55% of ad libitum.

As the result of slaughter experiments that we and our co-workers conducted with over 1,200 sheep, we concluded that the body composition within breed and sex of sheep which had been maintained in continuously positive energy balance was rigidly related to body weight, irrespective of feeding level and age of animal (Reid and White, 1978b). For

example, Burton and Reid (1969) reported that despite differences in the plane of nutrition imposed and in the body weights of sheep attained at given ages, weights of the chemical components of the whole, empty body were closely associated with body weight.

In most of the studies with cattle cited above, the energetic efficiency was not determined. As demonstrated in table 8, steers fed the anabolic agent, DES, had a greater rate of protein deposition but a lower energetic efficiency than their pair-fed mates that did not receive DES.

Rats fed by stomach tube in two meals a day the same amount of a high carbohydrate diet ingested by their ad libitum-fed mates gained the same amount of body weight, considerably more body fat and energy and less body protein and had a decidedly higher gross energetic efficiency (I. K. Han and J. T. Reid, unpublisbed data). Some of the observations made in that study are summarized in table 9.

The data cited, as well as other data (Pullar and Webster, 1972), imply that in the usual growth of animals, ME is utilized more efficiently in the net synthesis of fat than it is in the net synthesis of protein, and that the energetic efficiency is inversely related to the proportion of ME that is utilized for protein synthesis. A major future challenge is finding some means of reducing fat and increasing protein storage in meat-producing animals, yet improving their energy econ- omy. Whether, for example, animals might be re-engineered hormonally by immunological or other means to ply usual biological events needs to be examined.

Relative Efficiency of Protein and Fat Syn- tbesis. Attempts have been made by two methods to partition the amount of ME ingested above maintenance between body protein and fat. The methods are:

(1)

(2)

Total ME intake = a + b I fat + b2 protein, where ME intake is expressed as kilocalories/MBS/day, and fat and protein are expressed as grams/MBS/day or kilocalories/MBS/day, and

Total ME intake-ME for maintenance = a fat + b protein, where ME is expressed as kilocalories/MBS/day, the quantities of protein and fat are expressed as kilocalories/MBS/day and the recipro- cals of a and b represent the respective energetic efficiencies.


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NUTRITIONAL ENERGETICS OF LIVESTOCK

TABLE 9. EFFECT OF FREQUENCY OF MEALS ON BODY COMPOSITION AND GROSS EFFICIENCY OF RATS a

1407

Gain during a 14-day period

Body Gross Intake weight, g Fat, g Protein, g Energy, kcal efficiency, %b

Ad libitum 85.6 13.8 19.0 237 16.6 2 meals/day 83.5 30.5 12.8 359 25.9

aFrom I. K. Han and J. T. Reid (unpublished data).

bGross efficiency (%) = (body energy storage/dietary GE) • 100.

Table 10 presents a summary of estimates of the efficiency with which ME is utilized for the net synthesis of protein and fat in various animals, and it also itemizes several estimates of the theoretical efficiencies. In the estimation of theoretical efficiencies, the bases are that known biochemical pathways and substrates are operating, and it is assumed that nutrients replace each other in proportion to their free- energy yield computed as the pyrophosphate- bond energy of ATP. The hypothetical value for protein synthesis assumes that body protein is produced from dietary protein that has a biological value of 100%, i.e., protein containing essential and nonessential amino acids in the same proportions as those of the synthesized

protein. However, if nonessential amino acids were synthesized from carbon fragments and ammonia, the efficiency would be lower than that in the ideal situation. If an essential amino acid were lacking, other essential amino acids in excess of the required complement would be deaminated and the residue would be utilized to synthesize fat, resulting in a lower efficiency. In the synthesis of fat, the theoretical computations assume that glucose gives rise to acetyl coenzyme A by glycolysis. If glucose were produced from simpler compounds by gluconeogenesis, which to a great degree is the case with ruminants, the derived efficiency of fat synthesis would be lower.

As indicated in table 10, the estimated

TABLE 10. RELATIVE EFFICIENCY OF PROTEIN AND FAT NET SYNTHESIS IN ANIMALS

Efficiency of ME utiliza- zation for net deposition of:

Animal and conditions Protein Fat Source

(%)

Theoretical, general 90 to 93 70 or less Blaxter (1962); Schiemann (1963) Theoretical, ruminants 82 72 Baldwin (1968) Rats, growing 46 70 Schiemann et al. (1969) Rats, obese and lean, 48 to 84 days 43 65 Pullar and Webster (1974) Chickens, growing, over 365 g 51 78 Petersen (1970) Pigs, suckling, 2.5 to 8.5 kg 77 81 Kielanowski (1965) Pigs, 4 to 10 kg 84 81 Close etal. (1979) Pigs, 23 to 43 kg 54 70 Close et al. (1974) Pigs, 20 to 90 kg 43 77 Thorbek (1970) Pigs, 58 to 92 kg 34 74 Kotarbinska (1969)

Lambs, milk-fed, 5.5 to 10.5 kg 77 63 Kielanowski (1965) Sheep, 10 to 40 kg 33 82 Orskov and McDonald (1970) Sheep, 6 months mature 10 to 20 79 to 92 Rattray and Joyce (1976)

Calves, milk-fed, 58 to 155 kg 45 85 Kirchgessner et al. (1976) Cattle, 121 to 706 kg 34 69 IL Anrique et al. (unpublished data)


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1408 REID ET AL.

calorimetric efficiency of fat synthesis in animals is about the same as, or a little higher than, the hypothetical efficiency. However, the estimated calorimetric efficiency of protein synthesis in animals is considerably lower than the hypothetical value. This suggests that protein synthetic transactions in the whole animal are different from those theoretically expected to occur or that the energy costs of such ancillary events as protein turnover and resynthesis are not appropriately accounted for.

The existing data (table 10) indicate that the efficiency of protein synthesis in the very young pig, lamb and calf is considerably higher than that in older animals of the same species. Whether such trends are real or partly the result of the method of estimation is not known. Most data are based upon partitioning of the ME intake among maintenance and protein and fat storage by multiple regression analysis. The requirement for maintenance is usually large relative to that for growth, and changes in the maintenance requirement and in the proportion of energy partitioned into protein and fat are related to the degree of physiological maturity of the animal. Thus, maintenance and the storage of fat and protein are not genuinely independent variables. In a study designed to overcome to some degree autocorrelation among the measured variables, Pullar and Webster (1974) estimated efficiencies of protein and fat synthesis in lean and obese rats. Their estimates were similar to those determined by Schiemann et al. (1969), who used the partition method.

The amount of ME charged to maintenance has a pronounced effect on the estimated efficiency of protein and fat synthesis. In the experiment of Burlacu et al. (1976), the mean maintenance requirement of ME of pigs weighing 9 to 58 kg was 151 kcal/MBS/day, and the efficiencies of utilization of ME for protein and fat synthesis were 62 to 100%, respectively. The maintenance requirement is similar to that (148 kcal/MBS/day) determined for pigs under a body weight of 10 kg (Close et al., 1979), but it appears to be considerably higher than that of larger pigs (table 2). Accordingly, the estimated efficiencies of protein and fat storage are probably overestimated.

The evidence is overwhelming that the efficiency with which ME is utilized for net protein storage is considerably lower than that with which it is used for net fat storage (table 10), and that it is also much lower than the

theoretical efficiency of net synthesis. In the growing animal, the rate of accretion of muscle protein is small in relation to the turnover of protein. Other proteins turn over even more rapidly than muscle protein. According to Milligan (1971), the efficiency of peptide bond synthesis is only 3.9%, and because of turnover, the same protein would have to be resynthe- sized a number of times. Thus, the cost of turnover, resynthesis, synthesis of nonessential amino acids and transport of nutrients contrib- utes greatly to the low energetic efficiency of net protein synthesis as estimated in the whole animal.

Body Composition and Energetic Efficiency in Compensatory Growth. The body composition of growing sheep maintained in continuously positive energy balance is quite different from that of similar animals that have under- gone a period of compensatory growth to reach the same body weights. Figure 5 shows the growth patterns of sheep that were continuously fed and of those that were deprived so that they would lose about 30% of their body weight and then realimented in one study (Burton et al., 1974) of compensatory growth. Points 1, 2, 3 and 4 in the figure designate the weights and times at slaughter. The weights of chemical components and the amounts of energy in the sheep continuously ted and in those deprived or deprived and refed are summarized in table 11. It will be noted that sheep at 50 kg (group 1) that had been grown continuously from 40 kg had the same composition as those which had been grown continuously to 70 kg and then deprived of

70

v

6o

o

~, 50 ,,~

40<

L,,

~ I II(I

i I \ I I ~13

I i I 1 I 80 160 2qO 320 400

DURATION OF EXPERIMENT (DAYS)

Figure 5. Growth patterns of continuously fed and of deprived-refed sheep.


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1410 REID ET AL.

accounting is impaired, however, by the lack of knowledge of the energetic efficiency of some of the body functions of certain animals. Accordingly, the following section will concern the efficiency of protein production in various food products of animal origin expressed per unit of DE input.

DE Cost o f Protein Production. Since the chemical nature, the degree of refinement and, therefore, the DE value of the usual diet vary greatly among different kinds of animals, DE serves as a biologically equating parameter of dietary energy for all kinds of animals.

The data in table 12 represent the outputs (grams) of protein in various food products of animal origin per megacalorie of DE ingested. The values for fish (Rainbow trout) were estimated by Smith et al. (1978) and those for other animals were estimated by Reid and White (1978a). Since the efficiency of animal production increases with increasing inputs of DE per unit of time, the efficiency of protein production is shown for each of several rates of egg, broiler, pork, milk and beef production. The DE requirement of maintenance is constant per unit of metabolic size per day. Thus, with increasing inputs of DE, the proport ion of the

total input available for production increases. Accordingly, the efficiency of protein production increases with increasing production intensity.

The values in table 12 take into account the reproductive life span and the rates of sterility and mortal i ty found in average to good practice in the United States. The last value for pork and beef is based on the assumption that sterility and mortal i ty losses are nil. The protein represented by the carcass of the dams (mothers) was credited to the protein of their meat-producing offspring, milk or eggs; it was assumed that one of the offspring would replace the mother at the end of her productive life.

Because of the space needed to outline the conditions taken into account for all of the farm animal enterprises represented in table 12, only the considerations and assumptions concerning pork product ion will be cited here as examples of analogous considerations taken into account for the other animal enterprises. These are that: (1) the level of input, and therefore the rate of animal output , is corre- lated with the gross efficiency; (2) the production of 91-kg pigs at 6 months with a feed

TABLE 12. OVERALL EFFICIENCY WITH WHICH ANIMALS PRODUCE FOOD PROTEIN

Protein production efficiency, g

Food product Level of output and(or) intensivity of production protein/Mcal DE

Fish (salmonids)

Eggs

Broiler

Pork

Milk

Beef

1.5 kg feed/kg gain

200 eggs/year 236 eggs/year a 250 eggs/year

1.6 kg/12 weeks; 3.0 kg feed/1 kg gain a 1.6 kg/lO weeks; 2.5 kg feed/1 kg gain 1.6 kg/ 8 weeks; 2.1 kg feed/1 kg gain

91 kg/month; 6 kg feed/1 kg gain 91 kg/6 months;4 kg feed/1 kg gain a 91 kg/month; 2.5 kg feed/1 kg gain Biological limit (?) ; 2 kg feed/1 kg gain ; no losses

3,600 kg/year; no concentrates 4,944 kg/year ; 22% of energy as concentrates a 5,400 kg/year; 25 % of energy as concentrates 9,072 kg/year; 50% of energy as concentrates

13,608 kg/year; 65% of energy as concentrates

500 kg/15 months; 8 kg feed/1 kg gain a 500 kg/12 months; 5 kg feed/1 kg gain

Highly intensive system;no losses

40

10.1 12.6 13.7

11.9 13.7 15.9

5.0 6.1 8.7

12.1

10.5 12.4 12.8 16.3 20.5

2.3 3.2 4.1

aData represent approximately average USA management conditions.


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conversion rate of 4:1 represents the production rate under conditions of good practice; (3) the sow has the potential to produce five litters, each of 12 pigs, in 2�89 breeding years (assumption); (4) the amounts of feed ingested by the sow during rearing, gestation and lactation and those ingested by her meat- producing offspring during growth to a live weight of 91 kg constitute the total amount of feed chargeable to the carcasses produced; (5) the net production of protein includes that represented by the carcasses of the sow and her meat-producing offspring; (6) one of the sow's offspring would replace her in the herd (assumption); (7) at the first three rates of pork production listed, the rates of sterility and mortality are 12 and 25%, respectively (assumption); and (8) at the assumed biological limit, 12 ova would be fertilized and the sterility and mortality losses would be nil (assumption). (NOTE: Analogous assumptions and considerations underlie the data in table 12 for other animal enterprises.)

It is. clear that salmonids are much more efficient producers of protein than are livestock; however, they require a highly refined diet. The high efficiency of Rainbow trout seems attributable to their low energy cost of: (1) temperature maintenance, (2) locomotion, (3) reproduction (a 1- to 2-kg trout yields 3,000 to 6,000 offspring annually), (4) protein catabolism (ammonia, rather than urea or uric acid, is the principal end product of protein catabolism, and it is excreted chiefly through the gills), and (5) utilization of protein as an energy source.

Among the food products yielded by the common farm animals (table 12), efficiency is greatest for protein produced as milk, broiler meat and eggs; intermediate for that produced as pork, and lowest for that produced as beef. Within a given enterprise, protein-production efficiency increases with increasing intensity of DE input. As the losses from infertility and mortality are reduced, the efficiency increases; however, these effects have considerably less impact than does the level of energy input.

For a considerable part of their diets, or at least for land on which to grow their diets, chickens and pigs compete with humans. On the other hand, dairy cows can produce as much as 4,990 kg of milk on an all-forage diet, with an efficiency of about 12 g protein/Mcal DE. This yield is probably close to the upper limit possible on forages alone. Under the

conditions of good commercial practice, pigs yield 6.1 g protein/Mcal DE. Even at the extreme biological limit, the pig would produce only 12 g protein/Mcal DE at current rates of reproduction. As a producer of protein for human consumption, the pig occupies the most precarious position of all farm animals. This is because the pig produces a relatively low protein (about 13.5%) product at usual slaughter weights, it competes almost directly with humans for food or for land on which to grow it and the swine enterprise requires a high fossil energy subsidy. Nevertheless, the pig is an important scavenger animal in many parts of the world. Also, in some parts of the world, especially in the Far East, pigs are managed much less intensively than they are in the United States.

With the human population of the world increasing rapidly, the degree of competition between humans and animals for foodstuffs of plant origin is increasing steadily. Should foodstuffs of plant origin be available in excess of man's need, but not in sufficient amounts to produce the quantities of animal protein needed to satisfy man's present food consumption pattern, it will be necessary to decide which kinds of animals will be hus- banded in the future to produce man's food.

Protein production by the milking cow is at least as efficient as that by any other farm animal, excluding fish. Under most conditions, but especially when the milking cow eats at least enough forage to satisfy her energy requirements for maintenance, she will yield a greater gain in protein output per unit of concentrates (i.e., cereal grains and oilseeds) ingested than will any other farm animal. However, in some circumstances, the surplus concentrates ~ might be fed more effectively to fowls for meat production. For example, in some societies, milk is not used or it is not an important food, and in some peoples there is a congenital lack of lactase, which is essential to the digestion of milk carbohydrates. Also, under certain environmental conditions, or because of management problems involving disease or insect control, it may be more productive to feed available concentrates to fowl than to milking cows. Among the farm animals, fowl, especially chickens, are relativdy efficient protein producers, but they compete either directly with humans for foods or for the land on which to grow food.

Beef cattle of common British breeds


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1 4 1 2 REID ET AL.

produce only 4.1 g protein/Mcal DE ingested, even under an intensive production system in which (1) the dam is grown rapidly and bred at 8 months of age, (2) the meat-producing offspring produce 1 kg of gain per 5 kg of feed consumed and are slaughtered at 1 year of age, weighing 500 kg, and (3) there is no loss resulting from mortality or infertility. The relative inefficiency of beef production is attributable to the high energy cost of rearing and maintaining the breeding herd, of reproduction (long gestation period, primarily single births and a small number of meat producers generated by a given dam) and of the long feeding period required. However, although this meat production by cattle is relatively unaffected, these animals and other ruminants need not necessarily compete with humans for food. Also, under extensive management conditions, ruminants require a low fossil energy subsidy. For these reasons and because of the abundance of cellulose, the future role of ruminants as food producers seems secure despite the increasing human population pressure.

Present evidence indicates that at the same body weights, rapid-growing, slow-maturing cattle such as Holsteins contain more protein and less fat than slower growing, more rapidly maturing cattle such as Aberdeen Angus (R. Anrique, H. Ayala, S. Simpfendorfer, A. Fortin, A. F. Kertz, J. T. Reid and G. H. Wellington, unpublished data). Also, the efficiency of protein production is correspondingly higher per unit of DE input in rapid-growing, slow- maturing cattle. Within breeds, the females mature more rapidly and contain less protein and more fat at the same body weight than do the uncastrated males (figure 4). These characteristics are intermediate in the male castrate. Protein production efficiency also is-highest in uncastrated males, intermediate in male castrates and lowest in females.

Future Researcb CbaUenge. Analyses of livestock production systems are needed to determine the relative significance to protein production efficiency of the various inputs and the impact of improving them. For example, in swine production, what would be the integrated impact of improving the feed conversion rate from 4:1 to 2:1, of increasing the number of weaned pigs per litter from six to 12 or of producing 20 pigs per litter (by SUlYerovulation, with early weaning and by feeding a milk substitute diet) when all

other known components are integrated within the system?

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